Here’s a little SQL that may be helpful in certain troubleshooting situations. It can answer questions like:

• Which ORACLE_HOME is my database running from?
• Which server-side sqlnet.ora is used by my connection (TNS_ADMIN)?

Or more generally speaking: what’s the value of a given environment variable for my session’s server process.

with
  function get_env(var in varchar2)
  return varchar2
  is
    val varchar2(32767);
  begin
    dbms_system.get_env(get_env.var, get_env.val);
    return get_env.val;
  end;
select get_env('&var_name.') env_val from dual
/

Examples:

SQL> r
Enter value for var_name: ORACLE_HOME

ENV_VAL                                                                                                                                                                                                            
--------------------------------
/u01/app/oracle/product/ora12201                      

SQL> r
Enter value for var_name: TNS_ADMIN

ENV_VAL                                                                                                                                                                                                            
--------------------------------
/u01/app/oracle/network/admin

SQL> r
Enter value for var_name: PATH

ENV_VAL                                                                                                                                                                                                            
--------------------------------
 

Note, Oracle apparently intentionally redacts the value for certain environment variables, e.g. PATH.
But for the use cases above I’ve found it extremely useful…